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Schistosomiasis

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Epidemiological situation

Morbidity

Chronic schistosomiasis reduces the capacity of those infected to work and in some cases can result in death. In children schistosomiasis can cause anaemia, stunting and a reduced ability to learn. A review of disease burden estimated that more than 200 000 deaths per year are due to schistosomiasis in sub-Saharan Africa. The burden of disease due to schistosomiasis is underestimated. The Expert Committee on Prevention and Control of Schistosomiasis and Soil-transmitted Helminthiasis reviewed recent data on the morbidity and mortality due to schistosomiasis in sub-Saharan Africa and endorsed the new estimates.

The disease is caused primarily by parasite eggs, which are deposited by adult worms in the blood vessels surrounding the bladder or intestines.

The classical sign of urogenital schistosomiasis is haematuria (blood in urine). In women, urogenital schistosomiasis may present with a range of signs and symptoms including lesions of the cervix and vagina, vaginal bleeding, pain during sexual intercourse and nodules in the vulva. In areas endemic for urogenital schistosomiasis a large proportion of women may have female genital schistosomiasis (FGS). A recent WHO working group concluded that there is biological plausibility that female genital schistosomiasis is a risk factor HIV transmission to women. Genital schistosomiasis also affects men, inducing pathology of the seminal vesicles, prostate and other organs. This disease may also have other long-term irreversible consequences, including infertility. Bladder and ureteral fibrosis and hydronephrosis are common findings in advanced cases, and bladder cancer is also a possible late-stage complication.

Intestinal schistosomiasis has a nonspecific clinical picture of abdominal pain, diarrhoea, and blood in the stool. Liver enlargement is common in advanced cases, frequently associated with ascites and other signs of increased portal pressure. In such cases there may also be splenomegaly.

The Scientific Working Group (SWG) on Schistosomiasis held in 2005 characterized the clinical pathology due to the disease.

Transmission cycle

Transmission cycle of schistosomiasis

Causal agents of the disease are blood flukes (schistosomes). Their eggs leave the human body in urine (in urinary schistosomiasis) or faeces (in intestinal schistosomiasis), hatch in water and liberate larvae (miracidia) that penetrate freshwater snail hosts. After several weeks of growth and multiplication, cercariae emerge from the snails and penetrate human skin during contaminative water contact (wading, swimming, washing). Cercariae transform and subsequently migrate through the lungs to the liver where they mature into adult worms. These adult worms move to the veins of the abdominal cavity or of the urinary tract. Most of the eggs produced are trapped in the tissues but a proportion escape through the bowel or urinary bladder.

See a table with the different types of schistosome, their hosts and their geographical distribution.